1. Field of the Invention
Aspects of the present invention relate to a thin film transistor, a method of fabricating the thin film transistor, and an organic light emitting diode (OLED) display device including the thin film transistor.
2. Description of the Related Art
In general, a polysilicon layer can be applied to high field effect mobility circuits and high operating speed circuits. Polysilicon has the advantage of being applicable to complementary metal-oxide-semiconductor (CMOS) circuit structures and is widely used in semiconductor layers of thin film transistors. Such a thin film transistor is often used as an active device in active matrix liquid crystal displays (AMLCDs) and as a switching device or driving device of organic light emitting diodes (OLEDs).
Methods of crystallizing amorphous silicon to form polysilicon include solid phase crystallization, excimer laser crystallization, metal induced crystallization, and metal induced lateral crystallization. Solid phase crystallization is a method of annealing an amorphous silicon layer for a number of hours at a maximum temperature of about 700° C., which is the melting temperature of a glass substrate of a display device including a thin film transistor. Excimer laser crystallization achieves crystallization by irradiating an amorphous silicon layer with an excimer laser to locally heat it to a high temperature for a very short time. Metal induced crystallization induces a phase-change of an amorphous silicon layer into a polysilicon layer by placing the amorphous silicon layer in contact with, or doping it with, a metal such as nickel, palladium, gold or aluminum. Metal induced lateral crystallization involves inducing sequential crystallization of an amorphous silicon layer while a silicide formed by the reaction of metal and silicon expands laterally.
However, solid phase crystallization has the disadvantages of an excessively long processing time as well as the tendency of a substrate to warp due to lengthy, high-temperature heat treatment. The excimer laser crystallization has the disadvantages of requiring an expensive laser and forming crystallized surface protrusions that degrade the characteristics of a semiconductor layer—gate insulating layer interface.
At present, methods employing a metal to crystallize an amorphous silicon layer are being extensively researched, because they have the advantages of being able to produce crystallization at a lower temperature and within a shorter time, as compared to solid phase crystallization. Crystallization methods employing a metal include metal induced crystallization (MIC), metal induced lateral crystallization (MILC), and super grain silicon (SGS) crystallization.
One important factor determining the characteristics of a thin film transistor is a leakage current. Especially in semiconductor layers crystallized using a metal catalyst, the metal catalyst may remain in a channel region and thereby increase the leakage current. Accordingly, if the concentration of the metal catalyst remaining in the channel region is not controlled, the leakage current of the thin film transistor increases, and its electrical characteristics are degraded.